(a) Field of the Invention
The present invention relates to a method of methane steam reforming, using a nickel/alumina nanocomposite catalyst or a nickel/silica-alumina hybrid nanocomposite catalyst. More specifically, the present invention relates to a method of carrying out methane steam reforming using a nickel/alumina nanocomposite catalyst or a nickel/silica-alumina hybrid nanocomposite catalyst with an excellent methane conversion even under relatively severe reaction conditions of a high gas hourly space velocity or low steam supply.
(b) Description of the Related Art
Natural gas among the fuels usable as an alternative energy has the feature that its deposit is rich as well as it does not generate any side product. The natural gas whose main component is methane can be a good hydrogen source since the hydrogen to carbon ratio therein is higher than other fossil fuels. Based on this aspect, various techniques of producing hydrogen from methane have been developed, and typical techniques thereof include steam reforming of methane (SRM), autothermal reforming (ATR), partial oxidation, pyrolysis, etc.
Among the above, the steam reforming of methane is a strongly endothermic reaction wherein methane and steam are reacted to produce carbon monoxide and hydrogen.
The commercially available nickel-based catalysts used in such a steam reforming of methane have the demerits that they show a low activity since the size of active particles is not uniform and relatively large, they are vulnerable to sintering particularly when they are reacted at a high temperature, and also they show a significantly low durability due to the inactivation of catalyst progressed by the carbon deposition. In order to prevent such inactivation of catalyst due to the carbon deposition, excess steam is added to the methane steam reforming. In this case, the larger calorie is needed for the steam supply as the molar ratio of steam is higher, which ultimately lowers the energy efficiency during the reaction. In addition, the existing commercially available nickel-based catalysts have the disadvantage of low activity when they are applied to the steam reforming of methane at a high gas hourly space velocity.
The previous studies have been developed hitherto to overcome the above problems. For example, Korean Patent Laid-open Publication No. 10-2012-0122544 discloses a technique of improving the activity and life expectancy of a catalyst by providing an aqueous mixed solution containing a porous carrier, an alkali metal precursor, an alkaline earth metal precursor, and a titanium oxide, drying and calcining the aqueous solution to prepare a modified nickel catalyst wherein a nickel catalyst is combined with a composite consisting of an alkali metal or an alkaline earth metal and a titanium oxide, thereby enhancing the resistance against the inactivation of a nickel catalyst shown in the steam reforming of methane. Korean Patent Laid-open Publication No. 10-2014-0110660 discloses a technique of inhibiting the inactivation of a catalyst by adding an alumina support to a solvent wherein an organic acid is dissolved, impregnating and drying it to prepare the organic acid-treated alumina support, adding thus prepared alumina support to a solvent wherein a nickel precursor is dissolved, impregnating and drying it to prepare a core-shell structured nickel-alumina catalyst, on the surface of which is nickel uniformly distributed, thereby providing an excellent reactivity even under the condition of high gas hourly space velocity and inhibiting the sintering phenomena of nickel, which occurs inside the locally heated catalyst.
However, the previous techniques still show the problem of low methane conversion due to the low catalytic activity when the gas hourly space velocity is very high or the molar ratio of steam/methane (H2O/CH4) is low.
Furthermore, the nickel-based supported catalysts used in the previous methane steam reforming have been prepared via an incipient wetness impregnation method. However, the previous incipient wetness impregnation method requires a process of selecting a suitable solvent depending on the type of nickel metal salt used as the nickel precursor to obtain a catalyst wherein the nickel active particles are uniformed supported, and it also requires a process of repeating impregnation and drying at a high temperature to support the nickel active particles with a high loading amount. Such a process of using a solvent has the burden of risk at working, environmental pollution, treatment by excess solvent during the mass production of catalyst.
Also, it is needed to control the nickel metal particles to have a uniform and small size of 10 nm or less for the methane steam reforming.
Thus, there is a need for the development of a method of carrying out methane steam reforming with an excellent methane conversion even under relatively severe reaction conditions of a high gas hourly space velocity or low steam supply. It is also needed to develop a supported catalyst which can be prepared by a simple synthetic procedure and wherein the nickel active particles have a small size of 10 nm or less and are uniformly distributed with a high loading amount.
On the other hand, recently, a highly loaded with nickel, highly dispersed, and highly active nickel-silica yolk-shell catalyst that is very stable at a high temperature and shows a very high load of nickel in the level of 80-90 wt % has been developed. However, its use is limited due to such disadvantages as high manufacturing cost and difficulty in mass production.